Adenoviruses are double-stranded DNA viruses that have been isolated from a wide variety of avian and mammalian species, including swine. While the majority of adenovirus infections in swine are subclinical, porcine adenovirus (PAV) infection has been associated with encephalitis, pneumonia, kidney lesions and diarrhea. Derbyshire (1992) In: “Diseases of Swine” (ed. Leman et al.), 7th edition, Iowa State University Press, Ames, Iowa. pp. 225-227. Thus, there is a need for vaccines that will provide protection against PAV infection.
In addition to their potential ability to provide protection against PAV infection, PAVs could also be used as viral vaccine vectors, if insertion capacity can be determined, and appropriate insertion sites can be defined and characterized. It has been shown that PAV is capable of stimulating both humoral response and a mucosal antibody responses in the intestine of infected piglets. Tuboly et al. (1993) Res. in Vet. Sci. 54:345-350. Thus, recombinant PAV vaccine vectors would be especially useful, as they would be likely to be capable of providing both systemic and mucosal immunity to antigens encoded by native and/or recombinant PAV genomes.
Cross-neutralization studies have indicated the existence of at least five serotypes of PAV. Derbyshire et al. (1975) J. Comp. Pathol. 85:437-443; and Hirahara et al. (1990) Jpn. J. Vet. Sci. 52:407-409. Previous studies of the PAV genome have included the determination of restriction maps for PAV Type 3 (PAV-3) and cloning of restriction fragments representing the complete genome of PAV-3. Reddy et al. (1993) Intervirology 36:161-168. In addition, restriction maps for PAV-1 and PAV-2 have been determined. Reddy et al. (1995b) Arch. Virol. 140:195-200.
Nucleotide sequences have been determined for segments of the genome of various PAV serotypes. Sequences of the E3, pVIII and fiber genes of PAV-3 were determined by Reddy et al. (1995a) Virus Res. 36:97-106. The E3, pVIII and fiber genes of PAV-1 and PAV-2 were sequenced by Reddy et al. (1996) Virus Res. 43:99-109; while the PAV-4 E3, pVIII and fiber gene sequences were determined by Kleiboeker (1994) Virus Res. 31:17-25. The PAV-4 fiber gene sequence was determined by Kleiboeker (1995b) Virus Res. 39:299-309. Inverted terminal repeat (ITR) sequences for all five PAV serotypes (PAV-1 through PAV-5) were determined by Reddy et al. (1995c) Virology 212:237-239. The PAV-3 penton sequence was determined by McCoy et al. (1996a) Arch. Virol. 141:1367-1375. The nucleotide sequence of the E1 region of PAV-4 was determined by Kleiboeker (1995a) Virus Res. 36:259-268. The sequence of the protease (23K) gene of PAV-3 was determined by McCoy et al. (1996b) DNA Seq. 6:251-254. The unpublished sequence of the PAV-3 hexon gene (and the 14 N-terminal codons of the 23K protease gene) has been deposited in the GenBank database under accession No. U34592. The unpublished sequence of the PAV-3 100K gene has been deposited in the GenBank database under accession No. U82628. The sequence of the PAV-3 E4 region has been determined by Reddy et al. (1997) Virus Genes 15:87-90.
Adenoviruses have proven to be effective vectors for the delivery and expression of foreign genes in a number of specific applications, and have a number of advantages as potential gene transfer and vaccine vectors. See Gerard et al (1993) Trends Cardiovasc. Med 3:171-177; Imler et al. (1995) Hum. Gene Ther. 6:711-721. The ability of these vectors to mediate the efficient expression of candidate therapeutic or vaccine genes in a variety of cell types, including post mitotic cells, is considered an advantage over other gene transfer vectors. Adenoviral vectors are divided into helper-independent and helper-dependent groups based on the region of the adenoviral genome used for the insertion of transgenes. Helper-dependent vectors are usually made by deletion of E1 sequences and substitution of foreign DNA, and are produced in complementing human cell lines that constitutively express E1 proteins. Graham et al. (1977) J. Gen. Virol. 36:59-74; Fallaux et al. (1996) Hum. Gene Ther. 7:215-222; Fallaux et al. (1998) Hum. Gene Ther. 9:1909-1917. However, porcine adenoviruses do not replicate in human cell lines; hence these lines are unsuitable for the propagation of E1-deleted PAV vectors.
Though E1-deleted viruses do not replicate in cells that do not express E1 proteins, the viruses can express foreign proteins in these cells, provided the genes are placed under the control of a constitutive promoter. Xiang et al. (1996) Virology 219:220-227. Vaccination of animals with adenovirus recombinants containing inserts in the E1 region induced a systemic immune response and provided protection against subsequent challenge. Imler et al (1995) Hum. Gene Ther. 6:711-721; Imler et al. (1996) Gene Therap 3:75-84. This type of expression vector provides a significant safety profile to the vaccine as it eliminates the potential for dissemination of the vector within the vaccinee and therefore, the spread of the vector to nonvaccinated contacts or to the general environment. However, the currently used human adenovirus (HAV) based vectors are endemic in most populations, which provides an opportunity for recombination between the helper-dependent viral vectors and wild type viruses. To circumvent some of the problems associated with the use of human adenoviruses, non human adenoviruses have been explored as possible expression vectors. All vectors developed to date, except one (Klonjkowski et al (1997) Hum. Gene Ther. 8:2103-2115), contain an intact E1 region. Use of such vectors for gene therapy in humans and vaccination in animals is unsafe because they have the ability to replicate in normal cells, and they retain the oncogenic potential of the E1 region.
Recombinant PAV genomes containing heterologous nucleotide sequences have not yet been described. Similarly, sites where insertion of heterologous sequences would not interfere with the ability of a PAV vector to stimulate an immune response against a determinant encoded by an inserted sequence have not been identified. Consequently, the development of effective recombinant PAV vectors for use in immunication, expression systems and gene therapy, awaits resolution of these issues. Similarly, there is a need for improved adenoviral vectors lacking E1 replication and oncogenic functions, for expression of transgenes in mammalian cells.